The present invention relates to the suppression of modes of an aircraft on the ground.
Uneven deceleration of an aircraft during braking on landing is a problem. In particular the inventor has realised that it can cause the modes of the structure of the aircraft to be excited. These modes include both the normal modes of the aircraft structure, which flex the structure, and oscillations in the attitude of the aircraft, i.e. modes of rigid rotation of the aircraft. The flexing of the aircraft structures caused by the normal modes may cause undesirable fatigue damage. Excitation of the modes may also cause an uncomfortable ride for the passengers.
There are several sources of the uneven deceleration.
One source is the braking provided by the wheel brakes of the landing gears. The braking force provided by a ground wheel is equal to μFz where μ is the coefficient of friction between the tyre and the ground and Fz is the vertical load on the wheel. So, variation in the vertical load on a wheel leads to variation in the braking force. This variation in braking force in turn excites the modes (rigid rotational and normal modes) of the aircraft, to which, of course, the wheels providing the braking are attached. In turn the motion of the aircraft due to these excited modes affects the vertical load on the wheels, which can further excite the modes, and so on.
Also, the coefficient of friction can vary along a runway, and from runway to runway and from time to time, e.g. by being wet or dry. This therefore varies the braking force, again exciting the aircraft modes.
Another factor is that known braking systems often oscillate between a stable and an unstable behaviour, called skidding and releasing the brakes: when the wheels start to skid, the brake control system then prevents the skid. As the demand for braking is of course high skids soon reoccur. This switching back and forth changes the attitude of the aircraft and thus it is another cause of excitation of the modes of the aircraft.
The present invention seeks to suppress these aircraft modes and thereby alleviate the problems caused such as fatigue and uneven braking.